The threat of the detonation of a nuclear device in a major US city has prompted research aimed at providing more robust border surveillance for contraband nuclear material.
An article entitled “A Terrorist Threat—The movement of Black Market Nuclear Materials into the United States” dated November 2001 in the name of Gene R. Kelley from the article archives of the Nuclear Age Peace Foundation, PMB 121, 1187 Coast Village Road, Suite 1, Santa Barbara, Calif. 93108, USA, outlines the problem of surreptitious transport of special nuclear material. Kelly refers to some possibilities for moving this type of material as being as follows:                1)—superimpose the shipment of small, well-shielded packages on established drug and contraband routes.        2)—ship materials conventionally in well shielded, small containers through a surreptitiously network of widely dispersed handlers.        3)—man carrying many small quantities across the mostly porous borders of the United States.        4)—use diversified distribution techniques (routes and conveyances) by requiring multiple way-points and altering the characteristics of external shipping containers at each point.        5)—mix materials and legitimate products for routine deliveries.        
Kelley concludes that the formidable nature of the tasks required to detect and identify well packaged fissile materials renders the likelihood of detection in small quantities highly questionable.
The use of portal monitors at border crossing points is becoming routine for detecting smuggled nuclear materials. In many cases shielding can be used to obscure a nuclear signature. Conventional nuclear material detectors use high resolution gamma or X ray detectors.
Unshielded Kg quantities of highly enriched uranium can be detected with high reliability with 1 minute counting times by detecting gamma rays from the 238 U impurity. FIG. 1 of the accompanying drawings depicts example count data from a high resolution gamma ray detector used to detect Weapon grade uranium (WGU): 10% 238 U 90 % 235 U without shielding and with 5 cm and 2.5 cm of lead shielding, respectively. FIG. 1 indicates how self-shielding of nuclear material reduces count rates. In order to shield a threat object, about 5 cm thick lead, gold, tungsten, or other shielding material is required.
As indicated by FIG. 1 and additionally FIGS. 2 and 3, which illustrate simulations of X-radiography of 20 kg of uranium among automobile differentials using a fan beam of x-rays generated by 8 MV electron bhemstralung source. These simulations show that X-ray radiography can visualize objects, even in some dense, cluttered cargo, but definitive signatures of high z objects are confused by scatter backgrounds and transmission is insufficient for many cargos.
Having regard to the foregoing, the small amount of material needed to construct a nuclear device and the ease with which neutron and gamma ray signatures can be obscured with shielding makes robust border surveillance for contraband nuclear material difficult.